Among the semi-conductor material that may be used a detector materials silicon has many advantages such as high purity and low energy required for creation of charge carriers and also high mobility of charge carriers, all of which makes silicon predominating in the available semiconductor materials used primarily for radiation detectors. By implanting heavily doped layers as electrical contacts on top of low doping silicon and by applying a reverse bias to the junction to make the detector fully depleted, the radiation created charge carriers electron-hole pairs can be collected by the corresponding charge collecting electrodes.
There has been a considerable interest in silicon as the material for photon-counting detectors in particular for medical imaging. By far most detectors operate in an integrating mode in the sense that they integrate the signal from a multitude of x-rays and this signal is only later digitized to retrieve a best guess for the number of incident x-rays in a pixel. The last years so called photon counting detectors have emerged as a feasible alternative in some applications and commercially available mainly in mammography. The photon counting detectors have an advantage since in principal the energy of each interacting x-ray can be measured which yields additional information about the composition of the object, leading to improved image quality and/or a decrease in radiation dose.
Silicon has been used successfully in applications with lower energy as is for example outlined by M. Danielsson, et al., “Dose-efficient system for digital mammography”, Proc. SPIE, Physics of Medical Imaging, vol. 3977, pp. 239-249 San Diego, 2000. The main challenge with silicon is its low atomic number and low density which means it has to be made very thick for higher energies to be an efficient absorber. The low atomic number also means the fraction of Compton scattered x-ray photons in the detector will dominate over the Photo absorbed photons which will create problem with the scattered photons since they may induce signals in other pixels in the detector which will be equivalent to noise in those pixels.
There has been a continuous effort on evaluating the feasibility of employing silicon for high energy applications, such as computed tomography, as described in U.S. Pat. No. 8,183,535 B2 Mats Danielsson et al. “Silicon detector assembly for x-ray imaging”, Cheng Xu et al.: “Energy resolution of a segmented silicon strip detector for photon-counting spectral CT” Nuclear Instruments and Methods in Physics Research 715201311-17 and Xuejin Liu et al.: “Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications” Journal of Medical Imaging 23 2015 033502. An edge-on configuration of the silicon detector is described, with which the detection efficiency of silicon is increased significantly. Thin anti-scatter foil of a high Z element is attached to substrate to stop the scattered photons as a result of Compton scattering from reaching other silicon substrates.
Detectors having detector modules provided with collimators are illustrated in US2004/0251419 A1, Nelson et al. There it is shown how each detector in a strip detector is provide with a collimator. Adjacent strip detectors are separated by an air-gap.
Performance degradation from radiation-induced damages is a problem for any semi-conductor detectors. The relevant study on silicon has been carried out for decades. Particles traversing a silicon detector may interact with the material leading to ionizing or non-ionizing energy deposition. In both cases damage to silicon detector is possible. There are two types of radiation damages in silicon detectors, bulk damage and surface damage. The bulk damage due to the non-ionizing energy loss of incident particles is hard to happen for energy less than around 300 keV, whereas the surface damage causes most of the problems for silicon detectors used in the energy range of x-ray imaging from 40 keV to 250 keV. The surface damage is mainly introduced by the ionizing energy loss of charged particles or x-ray photons, which leads to the build-up of positive charges and traps in silicon dioxide and at the interface between silicon and silicon dioxide.
The success of silicon detectors using the planar processes relies strongly on the possibility to passivate the front-side surface with an oxide layer. Most often a silicon dioxide layer is grown thermally on silicon substrate by exposing silicon to an oxidizing ambient at elevated temperatures. When an x-ray interacts with a silicon detector, a cloud of charge carriers is released. The charge carriers created within silicon can be collected by charge collecting electrodes under an applied electric field, but those created within the silicon dioxide layer are trapped at the interface between silicon and silicon dioxide. Within several nanometer from the interface between silicon and silicon dioxide, the region is highly disordered, where the deep level defects are located. The deep level defects in silicon dioxide can trap holes and form fixed and positive oxide charges, which would cause some problems of the detector. There are some other kinds of defects in silicon dioxide and at the interface between silicon and silicon dioxide, discussed by Jiaguo Zhang: X-ray radiation damage studies and design of a silicon pixel sensor for science at the XFEL, and Jörn Schwandt: Design of a radiation hard silicon pixel sensor for x-ray science.
The defects induced by radiation impact electrical properties and mainly cause the following performance degradation of silicon detectors: increase of leakage current, increase of depletion voltage, increase of capacitance, formation of electron accumulation layer, decrease of breakdown voltage and charge loss near the interface between silicon and silicon dioxide. The electron-accumulation layer is relevant to the change of electrical properties of silicon detectors, and prevents the full depletion of a detector at the surface. The charge collection efficiency would also be affected by the electron-accumulation layer in the volume near the front-side surface of a detector. Consequently, there is a need in the art for semi-conducting detectors, in particular silicon detectors, which are less sensitive when exposed to x-ray radiation.